1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including a thin film transistor.
2. Description of the Related Art
A technique in which a thin film transistor (hereinafter also referred to as a “TFT”) is formed using a thin semiconductor film (with a thickness of several nm to several hundreds of nm, approximately) formed over a substrate having an insulating surface is known. Structures of TFTs can be roughly classified into two types. One is a top gate type, in which a gate electrode is formed over a channel formation region; the other is a bottom gate type, in which a gate electrode is formed under a channel formation region. TFTs are widely applied to electronic devices such as integrated circuits or active matrix liquid crystal display devices. In an active matrix liquid crystal display device, thin film transistors are used as switching elements of pixels. An amorphous silicon film, a polycrystalline silicon film, a microcrystalline silicon film, or the like is used for a thin semiconductor film of this thin film transistor.
An amorphous silicon film used in a thin film transistor is generally formed by a plasma-enhanced chemical vapor deposition method. A polycrystalline silicon film is formed by forming an amorphous silicon film by a plasma-enhanced chemical vapor deposition method (hereinafter referred to as a “PECVD method”) and crystallizing the amorphous silicon film. In one of the typical crystallization methods, an excimer laser beam is processed into a linear form with an optical system, and an amorphous silicon film is irradiated with the linear beam as the linear beam is moved.
The present applicant has developed a thin film transistor in which a semiamorphous semiconductor film is used for a thin semiconductor film (see Reference 1: Japanese Published Patent Application No. H4-242724; Reference 2: Japanese Published Patent Application No. 2005-49832; and Reference 3: U.S. Pat. No. 5,591,987).
A microcrystalline silicon film can be formed by a chemical vapor deposition method (hereinafter referred to as a “CVD method”) such as a PECVD method, or a physical vapor deposition method (hereinafter referred to as a “PVD method”) such as a sputtering method, and can also be formed by crystallizing an amorphous silicon film as shown in Reference 4 (Toshiaki Arai et al., “Micro Silicon Technology for Active Matrix OLED Display,” Society for Information Display 2007 International Symposium Digest of Technical Papers, pp. 1370-1373). The crystallization method in Reference 4 is as follows: an amorphous silicon film is formed, and then a metal film is formed over an upper surface of the amorphous silicon film; the metal film is irradiated with a laser beam that has a wavelength of 800 nm and is emitted from a diode laser; the metal film absorbs light, thereby being heated; and then, the amorphous silicon film is heated by the heat conduction from the metal film, thereby being modified into a microcrystalline silicon film. The metal film is formed to convert light energy into heat energy. The metal film is removed in a process of manufacturing a thin film transistor.